Storage devices or drives such as, hard drives, floppy drives, DVD drives CDROM drives and the like are widely used in electronic and computer systems. In fact, the use of such drives is increasing. Part of the reason for this increase in use is due to an increase in drive capacities. Such large capacities allow practical storage of information and data, such as video and/or multimedia files that were previously larger than the storage devices. However, as drive capacities increase, the sizes and tolerances of components used in drives must generally decrease. Further, the components used should be more reliable, accurate and smaller than previous generations of drives.
One important component used in storage devices is a DC motor. Motors are used for storage device operations such as rotating a disk or platter and positioning read/write heads. It is important that the motors are able to operate and be controlled at high and relatively constant speeds. Furthermore, it is important that the motors reduce or minimize effects such as torque ripple due to back Electromagnetic Forces (EMF) voltage and/or commutation timing errors.
There are two common approaches of driving three phase motors, using a pulse width modulation (PWM) scheme and using a linear current driver. The PWM scheme controls or drives the motor speed by driving the motor with short pulses. The pulses vary in duration to change the speed of the motor. The longer the pulse, the faster the motor turns and the shorter the pulse, the slower the motor turns. Thus, varying the length of the pulse controls the motor speed. However, a number of drawbacks or problems are associated with the PWM scheme. For example, the PWM scheme requires a larger area to be implemented. Furthermore, the PWM scheme generates a large amount of noise.
The linear current driver controls or drives the motor speed by driving the motor with a controlled current. The current is varied to control the speed of the motor. The larger the current, the faster the motor turns and the smaller the current, the slower the motor turns. A common way to build a linear current driver is to build a bridge for each motor phase. A high-side power transistor is used for the high-side and a low-side power transistor is used for the low-side for each phase. Thus, a total of six power transistors are used for a 3-phase motor. The motor commutates from one phase to the next as a result of the current driven through the phase windings by the power transistors.
Linear current drivers provide a number of advantages over comparable PWM schemes. Linear current drivers generally use less area and generate less noise than PWM schemes. Further, linear current drivers can be implemented in a fairly low cost manner.